Electrical ballast



Dec. 11, 1951 w. w. BROOKS ELECTRICAL BALLAST 2 SHEETSSHEET 1 FiledSept. 25, 1947 WV .o g m -O M 3m? m o 91% W8 A m .S M H. 9

mm H, 195? w. w. BROOKS ELECTRICAL BALLAST 2 SHEETS-SHEET 2 Filed Sept.25, 1947 lnvemtor. Wesley \/\/f Brooks by l W Has Attor'neig PatentedDec. 11, 1951 UNITED STATES PATENT OFFICE ELECTRICAL BALLAST Wesley W.Brooks, Fort Wayne, Ind., assignor to General Electric Company, acorporation of New York 1 This invention relates to electrical ballastsand more particularly to improvements in ballasts for electric dischargedevices.

An electric discharge device, as the term is here used, comprisesessentially a pair (or more) 5 of electrodes in an envelope containingan ionizable atmosphere. The discharge between the electrodes produces,among other well-known effects, radiation which may be visible orinvisible.

By making the envelope of glass, the device is widely used a lamp. Bycoating the glass, with a suitable phosphor invisible radiation isconverted to light as in the well-known fluorescent lamp. Electrically,all such devices have the unstable characteristics of an arc and henceit is necessary to ballast them with a suitable series impedance orimpedance network.

A well-known form of ballast is a high reactance transformer with acapacitor serially connected in its secondary circuit so as to make thatcircuit operate with leading current. By high reactance transformer ismeant a transformer which is specially constructed to have a higher thanordinary leakage reactance. The voltage ration of the transformer is sochosen as to transform any convenient supply voltage to the proper valuefor operating or starting the lamp. some lamps, of either the hot orcold cathode type, have a separate starter which, among other things,initiates a transient boosting voltage for starting the lamp so that thetrasnformer open circuit output voltage need not be enough to start thelamp. Other so-called instant start lamps, of either the hot or coldcathode type, have a ballast transformer whose steady state open circuitoutput voltage is sufi'icient to start the lamp.

One reason for operating the lamp circuit with leading current is thatit is more stable than a corresponding lagging current circuit. Anexplanation of this is that the non-sinusoidal lamp 40 current makes thelamp itself appear to have a lagging power-factor so that for the samecircuit applied voltage and lamp voltage a net capacitive ballast has ahigher impedance or higher ballasting voltage than a correspondinginductive ballast. Hence, the same variation in applied voltage and lampcharacteristics will not produce as great a change in lamp current witha net capacitive (leading) ballast as with an inductive (lagging)ballast. used to operate both hot and cold cathode lamps of the samesize at practically the same current. The term net capacitive is usedbecause such a ballast is not simply a capacitor but includes alsoinductive reactance, which in the case of a 55 Such a circuit can thusbe high reactance transformer is the reactance of the transformer. Thisinductive reactance controls the wave shape of the lamp current and italso provides the inductive kick which is initiated by the starter inthe starting of certain lamps.

An objection to leading current operated ballasts is their poor powerfactor. Heretofore this has been overcome by combining a lagging currentlamp circuit with the leading current lamp circuit as in a conventionalunitary two lamp ballast.

In accordance with this invention the lagging current lamp circuit withits poorer regulating properties is eliminated, while at the same timeachieving high power factor operation with one or more low leading powerfactor lamp circuits. This is accomplished by so constructing the highreactance transformer that it has an abnormally high exciting currentwhich effectively neutralizes the wattless component of the leading loadcurrent. There are many specifically different ways of obtaining thehigh exciting current but they all employ the principle of increasingthe reluctance of the part of the transformer core which is traversed byits magnetizing or exciting flux.

An object of the invention is to provide a new and improved electricballast.

Another object of the invention is to provide a high power factorballast for operating one or more electric discharge device circuitswith low power factor leading current.

A further object of the invention is to provide a high exciting currenttwo lead circuit high reactance transformer ballast for discharge lamps.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing Fig. 1 is a diagrammatic representation of a circuitembodying one form of the invention, Fig. 2 is a sectional view of oneform of transformer used in this invention, Fig. 3 is vector diagram forexplaining the operation of the invention, Figs. 4, 5, and 6 illustratemodified core structures suitable for use in this invention, and Fig. '7illustrates a modified structural relation of the circuit elements.

Referring now to the drawing and more particularly to Fig. 1 the dashedline I indicates diagrammatically the enclosing casing of the ballast.This is a two lamp ballast which operates a pair of discharge lamps 2and 3. The ballast comprises a high reactance transformer 4 and a pairtransformer comprises a magnetic core 1 on which are mounted windings 8,9 and I0. Magnetic shunts H and 12 are provided on the core forincreasing the flux leakage between the windings. Input terminals iiiare connected across the winding 8 which serves as the'primary windingof the transformer. windings 9 and III are autotransformer connectedwith the primary winding 8. Thus the lamp 2 is connected across thewindings 8 and 9 in series by way of a return condoctor [4 and thecapacitor 6 is serially connected in this circuit. Likewise, lamp 3 isconnected across the supply circuit l3, which is the ame as across theprimary winding 8, through the winding iii, the capacitor 6 and thereturn conductor it.

The reactance of the capacitors 5 and 6 is substantially higher, and inmany cases roughly double, the leakage reactance of the transformer sothat the secondary or lamp circuits of the transformer operate withleading current. In order substantially to neutralize this leadingcurrent the core 1 is provided with a high reluctance section in theform of an air gap l5 which is located on the part of the core which issurrounded by the primary winding 8. This high reluctance air gap causesthe primary winding to draw a relatively high low power factor laggingexciting current. a

Fig. 2 shows a shell type magnetic core construction in which gaps 16are in series with the magnetic shunts II and I! for giving them arelatively high reluctance so that they will not short circuit theexciting flux produced by the primary winding 8 and so that the opencircuit output voltage of the transformer will be substantiallyproportional to its turn ratio. The core consists of three separatepieces per lamination layer one piece being a straight central legmemher I! and the other two pieces being duplicate four-legged yokemembers I8. Instead of having a single gap [5 as in Fig. 1 the centerpart of the center leg I1 is provided with a series of gaps I5 whoseends are bridged by magnetic material at [9. These'bridged gaps areeasily made by punching slots in the lamination pieces.

The operation of the invention can best be explained by reference toFig. 3 in which the vector Ep represents the voltage of the primarywinding 8. The vectors 11.1 and 11.2 are the operating currents of thetwo lamps 2 and 3 whose sum is the vector In which is the total loadcurrent. In this diagram it should be understood that the currentvectors are referred to the primary side of the transformer. It shouldalso be understood that because of the non-sinusoidal currents taken bythe lamps the diagram is not rigorously accurate although it issufllciently accurate to illustrate the principle of operation of theinvention. It will be seen that the load current 11. leads the supplyvoltage E9 by the angle 0. This leading power factor angle is the resultof the voltage Ee of the capacitors 5 and 6. The leading load current inflowing through the series windings 9 and I0 causes both a change inmagnitude and displacement in phase of the series winding voltagerelative to the primary winding voltage so that the series windingvoltage is represented by the vector Es. The lamp operating voltage isthe vector ELamp- It will be observed that the lamp voltage and lampcurrent are slightly out of phase with the current lagging the voltage.This is not so much the result of any inherent reactance in the lampitself as it is the result of the current core.

wave shape which causes a wattmeter to indicate that the lamp consumesless watts than it does volt-amperes.

The abnormally high exciting current of the transformer which isproduced by the gap or gaps I5 is represented by the vector In which isshown lagging the primary voltage Ep by substantially degrees. The sizeof the gap or gaps i5 is so chosen that the vector sum In of In and ILis substantially in phase with the supply voltage Ep- As shown inth'diagram the effect of In is to reduce the magnitude of the loadcurrent from Ii. to In and also to reduce the input power factor of theballast from the angle 0 to the angle The modification shown in Fig. temploys a socalled forced core in which a straight leg portion 29 hasits ends forced into suitable notches in a generally rectangularlaminated yoke member 2! which is provided with the ma netic shuntmembers Hand !2. In this modification the yoke member at 22 isrestricted in cross section so as to provide the high reluctance sectionfor increasing the magnetizing current.

The modification shown in Fig. 5 (which now constitutes the subjectmatter of my divisional application Serial No. 124,877, filed November1, 1949, and assigned to the same assignee) difiers from that shown inFig. 2 in that instead of having bridged gaps l5|9 at the center of theleg portion of the core they are shown as bridged gaps 23 at the ends ofthe leg portion of the These bridged gaps therefore serve the doublepurpose of increasing the exciting current and limiting the transformersecondary flux due to the leading current in the series windings 9 andHi. This leading current tends to make the leakage flux of the windings9 and 50 more or less in phase with the exciting flux produced by thewinding 8 and without the gaps 23 the portions of the core traversed bythe leakage flux of the windings 9 and It! might saturate throughout themain body thereof and this would produce undesirable harmonics. Thislatter feature forms the subject matter of an application Serial No.731,559, filed February 28, 1947, in the name of Harold W. Lord andassigned to the present assignee.

In the modification shown in Fig. 6 there is a core type core whereasFigs. 2, 4 and 5 have illustrated shell type cores. Each laminationlayer of this core is made up of two punchings 24 and 25 with the endsof the outer legs of the members 24 shorter than the outer legs of themembers 25. The two members are reversed in alternate layers so as toprovide a lapped joint in the outer legs of the core. If desired abridge gap can be formed in the outer legs by making the outer legs ofthe center lamination pieces substantially equal in length and so thattheir combined length is less than the total length of the outer legs.This will produce gaps indicated by the dotted lines 26 in the same waythat the gaps l6 are provided in the magnetic shunts II.

and I2 and in the same way that the gap I5 is provided in the center legof the core.

In Fig. 1, the capacitors 5 and 5 are connected respectively to what maybe called the lamp ends of the secondary windings 9 and III. In somecases. a more economical arrangement is to connect them as shown in Fig.7 to the opposite or inner ends of the secondary windings 9 and I0. Thisis because the capacitors 5 and 6 can then be the two sections of aconventional double capacitor which is mounted in a single container 21.Such a double capacitor in a single case and with a midtap is ordinarilyless expensive for the same amount of capacitor volt amperes than aretwo separate capacitors of the same total rating. It is also unnecessaryto have three output leads extendingfrom the ballast casing I, and theconductor 14 may be a separate conductor which is not a part of theballast and which is merely wired in place when the ballast is connectedbetween its supply circuit and the lamps.

The flux density in the main body of all parts of the core issubstantially uniform in all of the modifications. The bridged portionsof certain of the gaps, which in effect are restricted sections somewhatanalogous to the restricted sections 22 in Fig. 4 will, of course,operate at higher flux density than the rest of the core and may evensaturate. However, in none of the modifications does the flux density ofthe main part of the core associated with the series windings 9 and Icarry materially different flux density from the main part of the corecarrying the primary winding 8.

Due to the low power factor leading current operation of the lampcircuits the operation of the invention does not depend in any way uponresonance in the lamp circuits.

It will of course be obvious that the invention does not depend upon theuse of more than one lamp and series winding and that therefore eitherof the two lamp circuits may be omitted if desired and the deviceoperated as a single lamp ballast. However, the two lamp construction ismore economical than two single lamp constructions and it results in abalanced and eflicient unit.

While there have been shown and described particular embodiments of theinvention it will be obvious to those skilled in the art that changesand modifications can be made without departing from the invention andtherefore it is aimed in the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric ballast comprising, in combination, a transformer havinginput and output circuits each including a coil on a unitary magneticcore, said core having a high reluctance magnetic shunt section betweensaid coils for increasing the series reactance of said transformer andhaving a high reluctance series section for decreasing the excitingreactance of said transformer, the reluctance of said shunt sectionbeing higher than the reluctance of said series section, and a seriescapacitor in said output circuit, the reactance of said capacitor beingof the order of at least twice the series reactance of said transformer,the exciting reactance of said transformer being such as substantiallyto neutralize in said input circuit the effect of the net capacitivereactance of said output circuit.

2. An electric ballast comprising, in combination, a transformer havinginput and output circults each including a coil on a unitary magneticcore, said core having a high reluctance magnetic shunt section betweensaid coils for increasing the series reactance of said transformerhaving a high reluctance series section for decreasing the excitingreactance of said transformer, the reluctance of said shunt sectionbeing higher than the reluctance of said series section, and a seriescapacitor in said output circuit, the reactance of said capacitorexceeding the series reactance of said transformer by approximately theexciting reactance of said transformer when referred to the output sideof said transformer.

3. In combination, an autotransformer having a magnetic core, spacedshunt and series windings on said core, high reluctance magnetic shuntsbetween said windings for giving such series winding a relatively highleakage reactance, at least one high reluctance section in the portionof said core traversed by the flux of said shunt winding for giving saidtransformer a high exciting current, the reluctance of said shunts beinghigher than that of said high reluctance section, and a capacitorconnected in series circuit relation with said series winding, saidcapacitor having a higher reactance than the leakage reactance of saidseries winding.

4. In combination, a balancedsingle phase shell type magnetic corehaving a center leg section and divided yoke sections, a primary windingon the middle of said leg section, extended autotransformer windings onthe opposite ends of said leg section, high reluctance magnetic shuntsseparating said windings, and separate capacitors connected respectivelyin series with seccndary windings which comprise respectively saidprimary winding in series with a different extended autotransformerwinding, the capa citive reactance of said capacitors beingsubstantially greater than the leakage reactance of said extendedwindings, the middle of said leg section having a high reluctancesection for causing said primary winding to have an abnormally excitingcurrent.

5. The combination as defined in claim 4 in which said capacitors are ina common container and have a common terminal.

WESLEY W. BROOKS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,088,803 Montroy Aug. 3, 19372,298,935 Freeman Oct. 13, 1942 2,310,742 Lord Feb. 9, 1943 2,312,768Mitchell Mar. 2, 1943 2,358,725 Mauerer Sept. 19, 1944 2,370,633 Boucheret al Mar. 6, 1945 2,429,604 Boucher et al Oct. 28, 1947

